Ultrashort laser pulses with pulse lengths from femtoseconds to picoseconds have found broad applications in many fields. For example, ultrashort laser pulses have been applied in the imaging of biomedical and solid-state media [1-9], which reveals subtle details of the media. By tightly focusing even low-energy pulses, ultrahigh intensities can be obtained, allowing ultra-precise micro-machining of materials from metals to dielectrics to the human cornea [10-12]. Ultrashort laser pulses also make possible the time-domain spectroscopic study of important ultrafast events in media from DNA to semi-conductors with unprecedented temporal resolution [13-16]. And by shaping these extremely short bursts of light into potentially very complex waveforms, it allows such exotic possibilities as the control of chemical reactions that cannot otherwise be controlled [17-22]. Essentially all these new techniques, including pulse shaping, operate best with the shortest pulse possible.
However, currently utilized laser pulses are far from optimally short. In practice, the laser pulses are longer than that emitted from the lasers, because of group delay dispersion (hereinafter “GDD”), which is caused when they pass through materials and/or optical elements. The GDD is a ubiquitous phenomenon in ultrafast optics. When a laser pulse propagates through dispersive media and/or optical elements, frequency components of different colors of the pulse emerge at different times, i.e., propagate at different velocities, thereby causing the resultant pulse to be stretched, reducing the pulse's peak power. Because the GDD is nearly always positive for wavelengths commonly available, the frequency components of the redder colors in the pulse always propagate faster than those of bluer colors, so that the redder colors precede the bluer colors in the pulse, in which case the pulse is said to be positively chirped. Chirped laser pulses are almost always undesirable because of the resulting increase of the pulse length and the reduction of the peak power.
The GDD effect may be compensated by using a pulse compressor, which may introduce a negative GDD. A conventional method for introducing a negative GDD is through angular dispersion. Martinez et al. [23] showed that the angular dispersion, regardless of its sign, yields a negative GDD. Therefore, simply propagating a pulse of light through a prism, or diffracting it off a grating, yields the negative GDD, whose magnitude depends on the propagation distance of the pulse of light. Unfortunately, a single dispersive element also introduces distortions in the pulse, such as angular dispersion and spatial dispersion. Adding another prism, identical to, but anti-parallel to the first one, a desired amount of the negative GDD could be achieved with no angular dispersion [24]. However, spatial dispersion remains in the single pair of prisms setup. The spatial dispersion can be compensated with an additional identical pair of prisms.
Referring to FIG. 7, a conventional pulse compressor 700 is shown, which utilizes four prisms 710, 720, 730 and 740 to compensate for the GDD effect. The pair of prisms 710 and 720 and the pair of prisms 730 and 740 are arranged symmetrically, while each pair of prisms 710 and 720 or 730 and 740 are aligned anti-parallel to one another. All the four prisms 710, 720, 730 and 740 are identical. For such an arrangement of the four prisms 710, 720, 730 and 740, when a chirped pulse 750 is input into the pulse compressor 700 through the first prism 710, the pulse compressor 700 outputs a corresponding compressed pulse 760 from the fourth prism 740, with compensations for angular dispersion, the spatial dispersion, and the pulse-front tilt. Additionally, the pulse compressor 700 can also compensate for the material dispersion of the prisms themselves. The pulse compressor 700 has been used in nearly all ultrashort-pulse applications for two decades.
Unfortunately, the pulse compressor 700 is as unwieldy as it is essential. For example, to vary the GDD over a wider range of values that can be obtained by simply translating a prism, the separations between the first and second prisms 710 and 720 and the third and fourth prisms 730 and 740 must be varied and maintained precisely equal, which involves several alignment parameters and an unwieldy set up. Also, the pulse compressor 700 has stringent alignment conditions, and, when not perfectly aligned, it yields an output pulse with residual amounts of spatio-temporal distortions [25] including angular dispersion, pulse-front tilt, spatial chirp, and one-dimensional beam magnification or demagnification that yields an elliptical output beam. It is also very inconvenient to tune in wavelength: if the input wavelength changes, all the prisms must be carefully rotated by the same amount, otherwise all of the above distortions occur. Additionally, to obtain a desired amount of the negative GDD, the prism separations can be quite large, so that the device is bulky.
The four-prism configuration of the pulse compressor 700 may be simplified to a configuration of two prisms through the use of a mirror after the second prism. The two-prism configuration of the pulse compressor is more compact and slightly easier to tune: only two prisms must be rotated by precisely the same amount. However, the two-prism design inherits most of the unwieldiness and propensity for spatio-temporal distortions of the four-prism pulse compressor.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.